Rubber compositions typically contain reinforcing fillers which may, for example, include synthetic amorphous silica (e.g. precipitated silica) as well as rubber reinforcing carbon black.
Various products are manufactured which have at least one component comprised of such rubber compositions such as, for example, tires.
In order to enhance rubber reinforcing effects of precipitated silica, a coupling agent is typically used in combination with the precipitated silica.
Such coupling agent typically contains a moiety (e.g. alkoxysilane group) reactive with hydroxyl groups (e.g. silanol groups) on the precipitated silica and another different moiety (e.g. polysulfide as a sulfur contributing moiety) interactive with elastomers containing carbon-to-carbon double bonds (e.g. diene-based elastomers).
Also, various functionalized elastomers are sometimes used in combination with precipitated silica reinforcement of the rubber composition where the functionalized elastomers contain functional groups which are interactive with the precipitated silica to thereby enhance a reinforcing effect of the silica for the rubber composition.
In practice, such functionalized elastomers are prepared by adding functional groups to the polymer chain of the elastomer, for example, adding a functional group to live terminal ends of the polymer chain during its formative monomer polymerization. The functionalized elastomer is then added to the rubber composition.
For this invention a very different approach is proposed, namely, addition of a dimercaptothiadiazole compound which contains a functional group to a rubber composition which contains precipitated silica and silica coupler where the functional group interacts in situ (interacts within the rubber composition) with the precipitated silica (e.g. with silanol groups on the precipitated silica) and silica coupler (e.g. with siloxane and sulfur moieties of a siloxane/polysulfide based silica coupler) within the rubber composition to form a crosslinked network within the rubber composition and, further, thereafter interacts with a later added sulfur to form additional crosslinks within the rubber composition.
While the complex mechanism, or combination of complex mechanisms, may not be completely understood, it is envisioned that the diazole group of the dimercaptothiadiazole is in a sense a functional group which can interact with the precipitated silica, (e.g. with hydroxyl groups on the precipitated silica) and possibly interact with a siloxane moiety of the silica coupling agent. It is further envisioned that the mercapto group(s) are in a sense functional groups which can interact with conjugated diene-based elastomers, (e.g. with carbon-to-carbon double bonds of a diene-based elastomer). It is additionally envisioned that the mercapto groups are in a sense functional groups which may further interact with the sulfur of a sulfur-containing silica coupling agent. In this manner, then, it is envisioned that a complex network results which aids in coupling the silica reinforcement to the rubber composition.
In practice, it is desired that the mercapto moiety of the dimercaptothiadiazole is protected from prematurely reacting with, for example, the diene-based elastomer and sulfur-containing silica coupling agent until its mercapto group becomes unprotected as a result of an elevated (higher) temperature sulfur curing of the rubber composition, and possibly in the presence of an amine based sulfur cure accelerator. Its mercapto group may be suitably protected, for example, by pre-treatment of the dimercaptothiadiazole prior to its addition to the rubber composition or by treatment of the dimercaptothiadiazole in situ within the rubber composition.
Such pre-treatment, or treatment in situ, of the dimercaptothiadiazole may be accomplished by treatment of the dimercaptothiadiazole to provide a protective group on the mercapto moieties. For example, the dimercaptothiadiazole may be treated with an aromatic carboxylic acid or an aromatic carboxylic acid chloride. In this manner, for example, it is envisioned that an aromatic thiol ester may be formed by an interaction of a mercapto moiety and the aromatic carboxylic acid. Such aromatic carboxylic acid or aromatic carboxylic acid chloride may be a compound comprised of at least one benzene ring in the molecule and at least one carboxylic group conjugated with the benzene ring. Representative of the many various aromatic carboxylic acids are, for example, and not intended to be exhaustive, benzoic acid, naphthenic acid, etc. Benzoic acid or benzoyl chloride are envisioned as being preferred. In this sense, it is envisioned that the benzoic acid reacts with the mercapto group of the dimercaptothiadiazole to thereby protect the dimercaptothiadiazole (protect the mercapto groups of the dimercaptothiadiazole from being chemically active with, for example, sulfur and sulfur-containing silica coupling agent) until the aforesaid sulfur curing of the rubber composition at an elevated temperature. Such sulfur curing may, if desired, be in the presence of an amine based sulfur cure accelerator.
It is therefore envisioned that a complex network of crosslinks are formed in situ within the rubber composition which is comprised of:
(A) crosslinks formed between the precipitated silica, silane coupler and dimercapto groups of the functional group-containing compound,
(B) crosslinks formed between sulfur, elastomer, and dimercapto groups of the functional group-containing compound, as well as
(C) crosslinks formed between the diene-based elastomer(s) and dimercapto groups of the functional group-containing compound.
It is believed that such redirection of precipitated silica reinforcement of a rubber composition, and resulting article of manufacture having a component comprised of such rubber composition is a significant departure from past practice.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise indicated.